The term "zone" is used in this application to identify a line or network of railway track, a line or network of roadway for other vehicles, or a section of some other form of traffic system for moving objects. The limits of the zone are defined by a plurality of "ports" through which vehicles or other objects may move as they arrive in or depart from the zone. These general words "zone" and "port" have been selected to minimize possible conflict with other terms having different and well established meanings in given transportation or conveyor systems, such as the term "block" as applied to railway systems.
A simple but important concept in traffic monitoring is the primary operation of determining when a zone is vacant and when it is occupied. A related secondary operation constitutes determination of the route followed by a particular stream of traffic in moving through the zone. Specifically, this entails the identification of the ports of arrival and departure. In a railway system or other vehicular system based upon wheel sensors that identify the movement or vehicle wheels through the sensors, it is obviously inadequate to limit the concept of identification of the presence of a wheel in the sensing zone of a sensor. Some other means is necessary for determination of the presence of a vehicle in the zone indirectly from the history of wheel movements through the zone ports. Similar considerations apply to other moving object traffic systems.
There are two basic ways to make the appropriate determination of object presence in the zone. The first entails a computation of object location from a determination of time and velocity data during transit through an arrival port. The second is by accumulated counting of wheel movements (or object movements) into and out of the monitored zone.
The time-velocity computation method, especially as applied to a vehicular transit zone, makes use of approximately known maximum distances between axles of various forms of rolling stock. If the vehicle speed is measured as the wheels pass through an arrival port, a relatively straightforward computation can predict the time limits between which successive wheel detections must occur. If no wheel passes through the access port within that time interval, a reasonable inference is created that there are no additional wheels available to pass through the port. That is, the traffic is gone.
However, this technique has two major flaws that render it quite unsuitable for general use. There is a substantial uncertainty in timing that is inherent in the system, an uncertainty made worse by any acceleration or deceleration. This inherent uncertainty translates into substantial doubt with respect to the location of the zone boundaries. In addition, the speed-time computation technique becomes unmanageable as the velocities approach zero. If reversal of movement of the objects is permitted, as can occur in almost any vehicular system, the velocity-time computation technique is rendered completely unreliable to the point of presenting a continuing danger to the moving objects and to any personnel involved.
In the second basic technique, with cumulative counting of arriving wheels (or objects) and continuous subtraction of departures from the arrival count, any accumulated count signifies the presence of traffic within the zone. A zero net count identifies a vacant zone condition. This technique has essentially no inherent uncertainty, but may be compromised by data input errors. Thus, the effectiveness of a zone monitor circuit based upon this second basic technique is dependent upon the effective elimination of probable errors.
Input data errors may include errors of omission, as when a sensor fails to detect the passage of a wheel or object into or out of the zone; conversely, spurious signals may indicate an arrival or departure through a port when no object is actually present. The consequences are false presence or vacancy determinations by the zone monitor. A secondary consequence may be an incorrect indication of the path of traffic movement through the zone.
Imperfect sensors and installations may cause an occasional object passage (or wheel passage) to be missed. A highly desirable goal for a zone monitor is to provide effective compensation for missed signals at a ratio of about one in several hundred; a monitor with this capability is relatively undemanding with respect to sensor condition and installation.
A false determination that the zone being monitored is vacant is highly undesirable and frequently dangerous. A total lack of response to arriving traffic is all but inconceivable. However, an initial failure to respond may cause a momentary false vacancy indication while traffic is actually present. In particular, a false vacancy indication may occur after a part of a traffic stream is within the zone or before it has completed departure from the zone. Cases of this kind may be viewed as an uncertainty in the location of the zone boundaries.
On the other hand, a false determination of traffic presence within the zone when the zone is in fact vacant is reasonably safe. This condition might be considered only a nuisance except that, if the monitor has no corrective mechanism, the false indication may persist indefinitely because the counting register must have an indefinitely long memory. Thus, an error of this nature can effectively disable the system.
A totally different kind of error can result from simultaneous application of arrival and departure signals to the zone monitor. This might appear to be statistically unlikely or even impossible, particularly when the output signals from the port sensors supplying data to the zone monitor are made exceedingly brief in comparison with the actual time of object passage through the sensors. In many systems, however, coincident arrival and departure signals can occur and may present a potentially major problem.
Thus, in railway systems and other vehicular systems, arrival and departure signals from the wheel sensors of the system are not uncorrelated in time. In a train of railway cars of identical dimensions, the sets of signals supplied by the wheel sensors form a pattern. Depending upon the car dimensions and the displacement between sensor locations, the pattern may cluster around possible coincidence and multiple near-coincident arrival and departure signals may occur. Rare errors brought about in this manner might well be tolerable except for an additional factor; unless positive steps are taken to avoid the difficulty, an otherwise acceptable system might generate errors much more serious than a single miscount when a coincidence situation develops. Thus, the responses may be indeterminate or difficult to predict. The overall effect may be what amounts to a latch-up of the system, a total counter reset, or other gross change.